1. Field of the Invention
The present invention relates to an apparatus for demodulating a multi-level QAM (Quadrature Amplitude Modulation) signal, and more particularly to one for orthogonally detecting a multi-level QAM signal, demapping the QAM signal after its digitization and then reproducing original code data.
2. Description of the Related Art
FIG. 8 is a block diagram showing a conventional apparatus for demodulating a multi-level QAM signal. Referring to FIG. 8, the orthogonal detector 1 orthogonally detects a modulated signal Sr modulated by a multi-level QAM modulation system and outputs analog demodulated signals of I and Q channels placed in an orthogonal relationship with each other. The filter 2 performs waveform-shaping of the outputs of the orthogonally detected signal and then outputs analog demodulated signals Si and Sq. The decision device 3 digitizes the analog demodulated signals Si and Sq and outputs these signals as digital demodulated signals D1i and D1q. And the demapping circuit 5 reproduces original code data Di and Dq from the digital demodulated signals D1i and D1q.
The multi-level QAM modulation system has a mapping circuit provided in a transmission side. For transmitting signals, the mapping circuit is disposed and transmits to be optimal in signal space such that a minimum free distance between code words can be maximum. The demapping circuit 5 is provided in a receiving side to perform an operation contrary to that of the mapping circuit in the transmission side. The demapping circuit 5 will be described more in detail later.
Referring to FIG. 9, in which signal point arrangements are shown for 32-level and 64-level QAM signals of a right-angle grid structure. White circles within an inner dotted line indicate a signal point arrangement for 32-level QAM signals. A signal point arrangement of 64-level QAM signals is indicated by black circles arrangement added to the white circles arrangement. The identification device 3 receives analog demodulated signals Si and Sq of I and Q channels arranged in such a signal-point manner, identifies these signals and then outputs digital values corresponding to respective signal points. In other words, as shown in FIG. 9, preset threshold levels L1, L2 and L3 are compared with each other concerning I and Q channels, high and low levels are identified and divided into 8 sections each, whereby digital demodulated signals D1i and D1q of bits I1, I2 and I3 and bits Q1, Q2 and Q3 are respectively outputted.
In the case of 32-level QAM signals as in the case of 64-level QAM signals, the identification device in which levels must be identified and divided into 8 sections of I and Q channels respectively. When 32-level QAM signals are affected by fading or noises in a transmission section and, as a result, signal points are positioned outside the dotted line shown in FIG. 9 and thus shifted from a normal signal point arrangement, signals of signal points that are not normally existent may be identified and detected by the identification device. If such data regarding the signal outside the dotted line is detected by the identification device and outputted to the demapping device, the demapping circuit cannot normally reproduce original code data and thus outputs indefinite data. A bit error then occurs and, consequently, reliability of demodulated data may be reduced.
The above phenomenon also occurs in the case of 128-level QAM signals. In FIG. 10, signal point arrangements for 128-level and 256-level QAM signals are shown. For convenience, only a first quadrant is shown. Also in FIG. 10, White circles within a dotted line indicate a signal point arrangement for 128-level QAM signals. For I and Q channels, in the first quadrant, an identification device for identifying levels and dividing the levels into 8 sections is required. Accordingly, by using the identification device, a signal point arrangement for 256-level QAM signals is also detected, the 256-level QAM signals being indicated by black circles outside the signal point arrangement for the 128-level QAM signals.
Thus, when a signal is shifted from a normal signal point arrangement, data regarding the signal outside the dotted line is also detected by the identification device and outputted to the demapping circuit. Consequently, a problem of demodulating indefinite data occurs similarly.
A similar problem occurs for a signal point arrangement of multi-level QAM signals of a honeycomb structure which is disclosed in JP(A) 1-500636 (1989). Specifically, for a signal point arrangement of a honeycomb structure shown in FIG. 11, since signal points exist to be detected outside a normal signal arrangement surrounded by a dotted line within a conversion input signal range of analog demodulated signals of I and Q channels, bit errors also occur.